1. Field of the Invention
This invention defines a composition useful as a drilling fluid. The composition is an oil-external micro-emulsion containing sodium petroleum sulfonate, water, hydrocarbon, bentonite, and optionally, cosurfactant.
2. Description of the Prior Art
Drilling fluids desirably have a variety of characteristics to facilitate exploitation of hydrocarbons in subterranean reservoirs. For example, the drilling fluid desirably lubricates and cools the drill string and bit during drilling operations, supports to some degree the weight of drill string and casing, prevents the sticking of drill pipe while in the borehole, keeps cuttings in suspension, carries the cuttings from the bit to the surface, releases the cuttings and any gas in the drilling fluid at the surface, provides well bore stability, prevents fluid from entering reservoir formations (i.e. prevents fluid loss), prevents formation fluids (oil, gas, water) from entering the well bore, provides corrosion protection, controls formation pressures, and aids in well-logging techniques (e.g. conducts electrical current needed for conventional electrical logging), etc. To perform the above functions, it is desired that the drilling fluid should:
1. have sufficient viscosity to support and transport cuttings and weighting materials; PA1 2. be fluid and pumpable, PA1 3. have sufficient gel strength to support cuttings and weighting materials when circulation stops; PA1 4. be stable at a wide range of pressures, temperatures, and shear rates; PA1 5. have the property to build filter cakes on the formation to prevent fluid loss; PA1 6. be chemically flexible so that the pH, etc. of the drilling fluid can be controlled; and PA1 7. conduct electrical current. The prior art does not know of a drilling fluid that will effectively meet all of the above requirements. PA1 1. Water-based mud which is the most common and which consists of water (fresh or salt), various clays, weighting materials, and other necessary additives; PA1 2. Oil-in-water emulsion muds generally composed of an emulsifier, oil, and water and other additives; PA1 3. Oil-based muds which contain refined hydrocarbons, e.g. diesel oil, a few percent water, and additives; and PA1 4. Water-in-oil emulsion muds which are often called inverted emulsion muds and consist of emulsifiers, water and oil, and other additives. The water-in-oil emulsion muds are the most popular for overcoming most of the serious drilling problems. A typical oil-external emulsion mud consists of diesel oil and water in a 7:3, 6:4, or 5:5 ratio to which may be added: (1) gelling and suspending agents such as clay-organo complexes; (2) emulsifiers such as anionic, sodium, and calcium salts of fatty acids, or cationic amine derivatives which supposedly are stable at high temperatures and in the presence of salt; (3) oil-wetting agents such as organic phosphates; and (4) filtration control agents such as clays or non-asphaltic organic colloids. The water-in-oil emulsion muds can be used as drilling fluids, completion fluids, or workover fluids and are generally preferred over water-based muds since the latter may cause problems with water-sensitive reservoirs. However, the water-in-oil emulsion muds are generally more expensive than the water-based muds. This added expense is generally justified to: PA1 1. prevent formation damage to water-sensitive reservoirs, PA1 2. maintain hole stability in heaving, dispersing, swelling, sticky, and sloughing shales by shielding the shales from free water; PA1 3. prevent hole enlargement and permit coring in water-soluble rocks, such as salt; PA1 4. secure full core recovery in unconsolidated formations; PA1 5. obtain cores which have not been flushed by water filtrate; PA1 6. maintain stable mud properties in deep and high temperature holes which require heavy mud; PA1 7. prevent differential-pressure sticking of drill pipe; PA1 8. prevent corrosion of drill pipe, casing, and tubing which reduces expensive workovers; PA1 9. permit the use of aluminum drill pipe for deeper drilling by protecting aluminum from salt corrosion; PA1 10. increase drilling rates by better lubrication and removal of shale cuttings without disintegration of cuttings; and PA1 11. generally permit the obtaining of lower density drilling fluids than water-based muds. Another advantage with the water-in-oil emulsion muds is that they generally can be reclaimed for later use; this reduces the overall cost. Oil-external emulsion muds have found widely accepted use in the Gulf Coast area; Pennsylvania reservoirs of Central Oklahoma, the Arctic North Slope, and reservoirs in Russia. The water-in-oil emulsion muds generally have a high resistivity to electrical current due to the oil; such adversely affects electrical logging techniques.
Four basic types of drilling fluids are presently available to the industry:
From a logging standpoint, it is desirable that the drilling fluid be sufficiently conductive of electricity to provide a path for the electric current to pass from the current electrode into the formation. If the fluid is too conductive, short-circuiting can occur between the current and the measuring electrode resulting in a greatly dampened, characterless, and misleading log.
There are many patents teaching examples of drilling fluids: U.S. Pat. No. 2,696,468 to Fischer discloses a conductive oil-base drilling fluid containing up to 10% water, an electrolyte and an emulsifying agent. Fischer teaches that above 10% water the fluid loses its desired properties and becomes an emulsion-base fluid. Fischer in U.S. Pat. No. 2,739,120 teaches similar drilling fluids containing non-ionic surfactants.
U.S. Pat. No. 3,111,491 to Atkins, Jr. et al teaches that oil-base drilling fluids lose their desired properties when the water concentration exceeds 10%.
U.S. Pat. No. 3,110,668 to Anderson teaches an emulsion fluid composed of guar gum derivatives and a surface-active agent (nonionic and cationic). Anderson teaches that emulsions containing anionic surfactants are undesirable unless stabilized in accordance with his invention e.g. in the presence of calcium, magnesium, sodium, and similar materials which are generally encountered during drilling operations.
U.S. Pat. No. 3,264,214 to Stratton teaches the use of metal petroleum sulfonates prepared from high viscosity oils as effective defoaming agents for drilling fluids. The drilling fluid can be an emulsion.